My 8 week old is constipated when she does go it's dark green poo with mucous. she's also fussing on the bottle.. any possible reasons why?

Not sure: Constipation is usually over-diagnosed by parents. Infants often strain. Constipation is when the stool is hard. Fussiness could be lactose in formula-or gas. Consider a formula with low levels lactose. Sometimes soy formula may help.

CONSTIPATION.....!: When stool is hard, big size and less often, it is constipation. Breast fed babies can have bm 6-8 times/day or every 6-7 days. If the stool is not hard, it is not constipation. If formula fed, green poo with some mucous is also within normal range- as long as it is not hard. For fussiness/colic, talk to her pediatrician, who may advise a hypo-allergenic formula, e.g. Nutramigen, alementum, etc.